In a phased array ultrasound imaging system, an ultrasound transducer comprises an array of transducer elements. The system includes n parallel channels, each having a transmitter and a receiver connected to one of the transducer array elements. Each transmitter outputs an ultrasound pulse into an object being imaged, typically the human body. The transmitted ultrasound energy is steered and focused by applying appropriate delays to the pulses transmitted from each array element so that the transmitted energy adds constructively at a desired point. The pulse is partially reflected back to the transducer array by various structures and tissues in the body.
Steering and focusing of the received ultrasound energy is effected in a reverse manner. The reflected ultrasound energy from an object typically arrives at the array elements at different times. The received signals are amplified, delayed and then summed in a receive beamformer. The delay for each element is selected such that the received beam is focused at a desired point. The delays may be varied dynamically so as to effect focusing on objects at progressively increasing depths as the ultrasound energy is received.
In a digital receiver, the signal from each array element is digitized by an analog-to-digital converter. The minimum conversion rate, or sampling rate, which is dictated by the Nyquist theorem, is twice the frequency of the highest component frequency in the received signal. Usually, the conversion rate is somewhat greater than the Nyquist requirement in order to permit a practical antialiasing filter.
In a digital ultrasound beamformer, it is straightforward to implement the delay function when the desired delay is an integer multiple of the sampling period. Such delay may be achieved by the use of a FIFO memory, a two port memory, a shift register or a similar memory device. However, it is often desirable to implement delays which are quantized in units smaller than the sampling period in order to obtain highly accurate steering and focusing. For example, assume a 5 MHz phased array transducer having a 100% fractional bandwidth, that is, a spectrum from 2.5 to 7.5 MHz. A sampling rate of 20 MHz satisfies the Nyquist requirement and provides a comfortable guard band for the antialiasing filter. For a 20 MHz sampling rate, a delay quantization of 50 nanoseconds (the sampling period) is easily achieved. However, to obtain a high performance beam plot, a delay quantization of about 12 nanoseconds is desired. It would be very expensive to increase the sampling rate to 80 MHz.
An alternative to increasing the sampling rate is to utilize a delay element that is capable of delays which are quantized in units smaller than the sampling period, thus providing delay interpolation between the sampling period. U.S. Pat. No. 4,170,766, issued Oct. 9, 1979 to Pridham et al., discloses a beamformer wherein signal samples are delayed relative to each other by fractional amounts of the sampling period using charge coupled device registers for storing analog samples. U.S. Pat. No. 4,787,392 issued Nov. 29, 1988 to Saugeon, discloses a technique for delay interpolation in an ultrasound system using two successively received ultrasound signals applied to an interpolator. U.S. Pat. No. 5,088,496, issued Feb. 18, 1992 to Bernard, discloses an ultrasonic imaging system wherein each channel of a beamformer includes a delay line and a circuit for delay interpolation. The circuit for delay interpolation comprises multiple digital filters, each having a different delay, connected in parallel. The output of one of the digital filters is selected to provide the desired delay.